Heat produced = V2t/JR kcal
For heater A, 500 = (V2 x (20 x 60))/(R x J) ......(i)
For heater B, 1000 = (V2 x (10 x 60))/(R x J) .....(ii)
From Eq. (i) and (ii), we get, R = 2.5 Ω.
When the two heaters are connected in series, let H be the amount of heat produced in kcal.
Since combined resistance is (10 + 2.5) = 12.5 Ω, hence
H = (V2 x (2 x 60))/(12.5 x J) .....(iii)
Dividing Eq. (iii) by Eq. (i), we have H = 100 kcal.